Hyperthermia causes changes in expression of TGF-β mRNA and protein in cultured cardiac cells, as well as in the heart in vivo. 12 h after hyperthermia, primary cultures of neonatal rat cardiomyocytes show a two- to threefold decreased expression of TGF-β mRNAs which returns to control levels by 48 h after heat shock. In cultures of cardiac fibroblasts, expression of TGF-β mRNAs increases 5-25-fold, 12-48 h after heat shock, while fetal bovine heart endothelial cells show little change in TGF-β expression after hyperthermia. In each case, mRNAs for TGF-βs 1, 2, and 3 are regulated similarly. Hearts isolated from rats exposed to hyperthermia show an initial 20-fold decrease in TGF-β 1 and 3 mRNA levels which return to control levels by 24 h and subsequently are elevated two- to threefold above normal 48-72 h after heat shock; there is little change in TGF-β2 mRNA. Expression of immunoreactive TGF-β1 and 3 protein, localized intracellularly in myocytes, follows the same pattern as the mRNA expression. By 72 h, some myocytes show hyperstaining for TGF-β1. Staining for extracellular TGF-β1/3 exhibits the opposite time course, being most intense 3-6 h after heat shock and returning to control levels by 48 h. The increase in TGF-βs after hyperthermia could play a role in mediating the reported cardioprotective effects of heat shock.